Class/density
(g/cc) 		Frictional resistances of
tap drilling
Mean (SD)
D1/0.48 16.89 (5.79)a
D2/0.32 4.31 (1.1)b
D3/0.16 1.38 (0.2)c
D4/0.08 0.92 (0.16)d

a-b, a-c, a-d, b-c, b-dStatistically significant.
c-dNot statistically significant.
All data were analysed at the 5% significance level using one-way analysis of variance (ANOVA) followed by Tukey's post-hoc test.
SD = standard deviation.